Y. Dong et al.
Carbohydrate Polymers 260 (2021) 117815
prepared via fluorinated compounds modified cellulose. The catalyst
showed excellent catalytic activity and selectivity in the synthesis of
in sustainable process.
2. Experimental
2
-methoxy-4-methylphenol under atmospheric hydrogen pressure.
As the most of abundant natural biopolymers second only to cellulose
and the N-deacetylated derivative of chitin, chitosan (CS) has received
2.1. Materials and chemicals
growing concerns in the development of different organic trans-
formations (Moln a´ r, 2019). Depending on the presence of reactive ꢀ NH
Chitosan (low molecular weight, 80–95 % deacetylated), mono-
chloroacetic acid, acetic acid and methanol were obtained from National
Medicine Group Chemical Reagent Co., Ltd. Vanillin was purchased
from Aladdin Industrial Corporation. Deionized water was used in this
2
groups along with ꢀ OH groups on the macromolecule backbone, this
low-cost and nontoxic biopolymer not only possesses unique affinity for
most transition metal species, but also achieves functionalization via
various modifications, making it a desirable solid support (Hardy,
Hubert, Macquarrie, & Wilson, 2004; Nasir Baig, Vaddula, Gonzalez, &
Varma, 2014). For example, Zeng et al. (Zeng, Qi, Yang, Wang, & Zhang,
2 4
study. Sodium tetrachloropalladate (Na PdCl ) was synthesized ac-
cording to the literature (Baran, Baran, & Mente s¸ , 2018). All other
materials were of analytical grade and directly used as received.
2
014) have reported chitosan directly cross-linked by Pd (II) cation
membranes, which was highly efficient in Heck cross-coupling re-
actions. In another study, a chitosan supported heterogenous catalyst via
2.2. Instrumentation
palladium anchored on the Fe
prepared, which exhibited excellent catalytic performance in the syn-
thesis of benzonitriles with K [Fe(CN) ] (Baran, 2020). Accordingly,
3
O
4
/chitosan/pumice hybrid beads was
Fourier transform infrared (FT-IR) spectra were recorded with a
Bruker VERTEX70 spectrometer using KBr pellets. XRD measurements
were performed at room temperature on a Rigaku SmartLab-SE X-ray
powder diffractometer. TGA experiments were carried out on Perki-
4
6
this advantageous biopolymer is capable of being easily modified and
exploited as a matrix to produce the effective heterogeneous catalysts.
Currently, Schiff bases and their metal complexes have witnessed
constant development in catalysis field and other applications e.g.
antibacterial (Rasool, Hasnain, & Nishat, 2014), liquid crystal materials
◦
◦
nElmer thermogravimetric analyzer from 40 C to 800 C at a heating
◦
ꢀ 1
rate of 10 C min in air flow. Scanning electron microscopy (SEM)
image of the samples was taken using the instrument (Hitachi SU 8010).
Transmission electron microscopy (TEM) images were acquired on a
1
(
Shukla, Rao, & Rakshit, 2003) and semiconductor (Aly & Khalaf, 2000).
Tecnai G2 F30 working at 200 kV. H NMR spectra of Suzuki products
As a versatile ligand containing HC=N moiety, nontoxic Schiff bases are
prepared from the condensation of amino groups with aldehyde or ke-
tone, and retain metal species through excellent electronic properties
were obtained from a Bruker AV-400 MHz instrument in CDCl using
3
1
3
tetramethylsilane (TMS) as the internal reference. The Solid state
C
CP-MAS NMR spectrum was accumulated on a Bruker Advance III-600
MHz spectrophotometer system. X-ray photoelectron spectroscopy
(XPS) detections were collected via an AXIS-ULTRA DLD-600W Instru-
(
Borisova, Reshetova, & Ustynyuk, 2007). Several studies have been
reported in literature regarding Schiff bases modified biopolymers for
catalytic applications. For instance, Leonhardt and colleagues (Leon-
hardt et al., 2010) synthesized Schiff bases modified chitosan from sal-
icylaldehyde and 2-pyridinecarboxaldehyde and their palladium
complexes were utilized for the preparation of desired products with
high yields in Suzuki reactions. While satisfactory results were obtained
in above studies, there is still much demand for developing novel Schiff
bases modified chitosan. The novel catalysis system could provide more
coordination sites with metal in order to enhance stability and prevent
ment with Al K irradiation. The elemental contents of palladium in the
α
catalysts were determined with a Thermo Scientific ICAP 7200 SERIES
inductively coupled plasma-optical emission spectrometry (ICP-OES).
2.3. Preparation of OCMCS-SB
At first, 1.5 g chitosan was dissolved in 2 % acetic acid solution (75
mL; V:V). Next, methanol (60 mL) was added to the chitosan solution
◦
metal
leaching
efficiently.
Vanillin
(4-Hydroxy-3-methox-
and the solution was stirred at 75 C under N atmosphere for 4 h. Then,
2
ybenzaldehyde), an aromatic aldehyde compound, is considered as a
commonly used food flavoring agents and is extracted from pods of
Vanilla planifolia (Karathanos, Mourtzinos, Yannakopoulou, & Andri-
kopoulos, 2007). This compound has been extensively employed in
many industrial procedures containing food, beverages, perfumes and
nutraceuticals (Mourtzinos, Konteles, Kalogeropoulos, & Karathanos,
the solution of Vanillin (4-Hydroxy-3-methoxybenzaldehyde, 2.685 g,
17.7 mmol), which was dissolved in methanol (25 mL), was added into
the reaction mixture. The resulting mixture was refluxed for 12 h
overnight. After the addition of the solution of 7.5 g monochloroacetic
acid in 10 mL of methanol drop by drop, the mixture was allowed to
◦
continue at 60 C for 5 d. The resultant solid was filtered and washed
2
009; Sinha, Sharma, & Sharma, 2008). Consequently, the exploitation
with methanol and ethanol until the washing fluid got clarified. Finally,
◦
of Vanillin functionalized chitosan as a novel support for stabilizing
palladium is an alternative candidate to explore as polymeric support for
future applications.
the solid was dried under vacuum at 60 C to give the yellow product
(OCMCS-SB).
In light of our interest in the production of polysaccharide supported
catalysts (Dong et al., 2020), in this study, we designed and prepared
Vanillin functionalized carboxymethylation chitosan as the sustainable
substrate to support palladium species for the preparation of Pd complex
2.4. Preparation of OCMCS-SB-Pd(II)
OCMCS-SB (0.6 g) was dispersed in 20 mL deionized water at room
temperature. After adding the aqueous solution of 0.3 g Na PdCl (20
2
4
◦
[
OCMCS-SB-Pd(II)] (OCMCS: Vanillin functionalized carbox-
mL), the mixture was reacted at 50 C for 24 h. Following the completion
of reaction, the obtained mixture was cooled down and filtrated. The
filtrate was rinsed with deionized water and dried under vacuum at 60
ymethylation chitosan; SB: Schiff Base). The Pd complex was applied for
Suzuki-Miyaura reactions in eco-friendly media and the tests indicated
that it could effectively catalyze and accelerate the formation of biaryl
compounds with high reaction yields under mild condition. Owing to the
interaction of trifunctional entrapping sites (Schiff base, hydroxy and
carboxy groups) with palladium, higher palladium content and yields
were obtained than those of CS-Pd(II) in the Suzuki reaction. In addition,
the reusability experiments showed that OCMCS-SB-Pd(II) was reused
up to five times with acceptable yields and negligible metal leaching.
Also, a reasonably trifunctional complex structure was proposed.
Noticeably, this study presents a novel strategy to prepare eco-friendly
polysaccharide materials, which are expected to have potential utility
◦
C to afford the light yellow product [OCMCS-SB-Pd(II)].
2.5. General procedure of Suzuki-Miyaura cross-coupling reaction of aryl
halides with aryl boronic acids
In a brief, aryl halide (0.50 mmol), arylboronic acid (0.75 mmol),
and K CO (1 mmol) were dissolved in the mixture of ethanol (3.0 mL)
2
3
and water (2.0 mL), followed by addition of OCMCS-SB-Pd(II) catalyst (2
mg, 0.46 mol% Pd, referred to aryl halide). The solution was magneti-
◦
cally stirred at 50 C for a certain period of time. TLC was adopted for
2